Multiple access communication systems basically provide a common information network for transferring digital message signals among computers, terminals and related equipment. In prior art systems, there are a number of protocols used to accommodate the various equipment in the network and provide them with access to a communications bus. Such protocols include simple polling, priority request, contention, carrier-sensing, carrier-sensing with collision-detection, token-passing, and cyclic time-division.
In the polling type system, a central controller sequentially polls each of the subscribers (includes computers, terminals or related equipment), offering each an opportunity to access the network when available. In priority request systems, subscribers ready to transmit a message make a request, and are granted access to the network according to priorities established by an arbiter at a central controller. In contention systems, subscribers may transmit messages at random times and retransmit after a random delay in the event two or more simultaneous transmissions destroy the messages. The error caused by simultaneous transmissions may be detected by lack of acknowledgement from the destination subscriber or by monitoring the signal on the network for two or more simultaneous transmissions. In carrier-sensing systems, subscribers may transmit only when the network is idle and retransmit after a random delay if no acknowledgement is received from the destination. In carrier-sensing systems with collision-detection, subscribers may transmit when the network is idle, monitor the signals on the network, and stop transmission and retransmit after a random delay if two or more simultaneous transmissions are detected. In token-passing systems, a subscriber may transmit when it holds a special message called a "token" and at the end of its transmisson, pass the "token" to the next subscriber in a predetermined sequence. In cyclic time division systems, a subscriber may transmit in slots assigned to it from regularly occurring time slots in a repetitive framed sequence. The assignment of slots to various subscribers in the network may be centrally controlled or may be distributed among the subscribers. Further, each slot may be assigned to only one subscriber or be assigned to more than one subscriber in which case the subscribers may contend for transmission in a time slot.
The polling and priority request approaches have been applied to multiple access communication systems in the prior art whereby a central subscriber controls the bus access. However, such systems are typically characterized by a rigid formatting of messages and an inflexible set of system constraints controlling the time periods at which the various remote subscribers may gain access to the communication path. In addition, the various data rates at which the individual remote subscribers may transmit message signals are hard-wired into the system to define predetermined portions of the channel bandwidth which are allocated to each of the remote subscribers.
The contention, carrier-sensing and carrier-sensing with collision-detection protocols have been applied in the prior art to accommodate low duty cycle, or "bursty", subscribers, like terminals. In these multiple access communication systems, the entire bus bandwidth is available to a subscriber in the network. Therefore, such systems do not permit control of access to the network, which is needed to give preferential access to higher priority subscribers. Further, as the number of subscribers is increased or high duty cycle subscribers, like computers, are placed on the network, the bandwidth wasted due to collisions and the time required to transmit a message from a source to a destination subscriber increases. These factors seriously deteriorate the performance of the network. Also, in carrier-sensing and carrier-sensing with collision-detection approaches, a subscriber can determine the end of a message only after actually receiving the end of an on-going message. This forces the inter-message pause and the duration from the start of a message during which a collision could occur to be dependent on the maximum separation between the subscribers. Therefore, the utilization of the bus bandwidth is reduced as the length of the network increases.
The token-passing method has been applied in the prior art to accommodate high duty cycle subscribers, like computers. However, such multiple access communication systems do not optimally accommodate a large number of low duty cycle subscribers. Further, complex hardware is needed, in any network topology except "ring" topologies, to control the passing of the token and to recover from failure of a subscriber.
The cyclic time-division system has been applied in the prior art to permit control of access to the network and to accommodate both low and high duty cycle subscribers. However, this multiple access communication protocol requires a central controller to synchronize all the subscribers and any outage of the controller causes failure of the entire system. Further, the fixed message length results in underutilization of bus bandwidth assigned to subscribers with short messages and increased complexity for transmission of messages longer than a slot.
Typically, in operating environments, a system is required to accommodate high and/or low duty cycle subscribers having short and/or long messages. Further, a network should be adaptable to changes in an operating environment.
Accordingly, it is an object of the present invention to provide a multiple access communication protocol which not only optimally accommodates different operating environments, but is also adaptable to changes in an operating environment.
It is another object of the invention to provide a multiple access communication system which can optimally accommodate, on the same network, a combination of high and low duty cycle subscribers having short or long messages.
A further object of the present invention is to provide a multiple access communication system which will permit: optimal accommodation of high duty cycle subscribers having variable length messages without actually passing a token; optimal accommodation of low duty cycle subscribers having variable length messages including a capability to control access to the network based on subscriber priority; network operation not dependent on proper functioning of any single controller; and a system which, while allowing variable message lengths, makes the inter-message pause independent of the length of the network and the distance between the subscribers for topologies that use two one-way links.